The present invention relates to instrumentation for use with an apparatus known as a drill string compensator or "D.S.C." Drill string compensators are typically employed on a drilling vessel to compensate for relative vertical movement between such vessel and a drill string being run therefrom caused by heaving of the vessel on the water. For example, a drill string may typically be supported on the traveling block of a block and tackle assembly, and the operator will seek to lower this traveling block at a feed rate corresponding to the penetration rate of the drill bit carried at the lower end of such drill string. However, since the traveling block is in turn carried by the drilling vessel, heaving of the latter can interfere with a proper rate of movement of the drill string. Accordingly, a drill string compensator is preferably interposed between the traveling block and the drill string. For example, a typical compensator includes a pair of parallel piston and cylinder assemblies in which the pistons are supported by compressed air. The drill string may be suspended from the piston rods, while the cylinders are fixed with respect to the drilling vessel and therefore movable therewith. Thus, the piston and cylinder assemblies, in essence, form cushioned telescoping joints which permit the vessel to heave without interfering with the position of, or weight on, the drill string. By controlling the pressure of the air supporting the pistons within their cylinders, such compensators can also be used to control the weight on the drill bit.
As the drill bit penetrates the earth the operator seeks to lower the traveling block at the rate of penetration and also to maintain the pistons in generally centralized positions within their cylinders. To accomplish this, some type of indicator means must be provided to enable the operator to determine, as nearly as possible, the position of the piston within the length of the cylinder as it moves.
One prior indicator subsystem utilized a member, movable with the piston, and having a series of bores therethrough. Means were provided for emitting light so as to shine through these bores as they passed a given point. A counter device having means for detecting light passing through the bores and thereby counting the number of bores which passed the reference point would emit a corresponding signal for the operator. One problem with this system is that it is not as accurate as might be desired, since it can only detect increments of movement and thus does not continuously indicate the exact position of the piston. Another problem associated with this incremental counting system is that it has no permanent memory. Thus, if electrical power is momentarily interrupted, the proper position indication will be lost. It is then necessary to reset the read-out by moving the piston to one extreme end of its stroke. This resetting procedure is extremely troublesome and undesirable.
Other position indicator systems employ a rotary electrical potentiometer connected to a rotatable drum mounted on one of the relatively moving members of the drill string compensator, e.g. the cylinder. A wire is wound on the drum and its free end attached to the other member of the compensator, e.g. the piston rod. As wire is pulled from the drum by the piston rod, the drum is caused to rotate. This operates the potentiometer which emits an electrical signal which may be translated into a read-out value generally indicative of the piston position. A spring is usually employed to rewind the wire as the compensator movement is reversed. Systems of this type are frequently troubled by such problems as breakage of the drive wire and spring failure.